1. Field of the Invention
The present invention relates to a linear scale, a linear motor, and a linear motor controller.
2. Description of the Related Art
A semiconductor production apparatus or other similar type of apparatus is equipped with an elongate moving-magnet type linear motor for conveyance.
The moving-magnet type linear motor is provided with a base frame adapted to set a given linear movement direction, an armature mounted to the base frame, a rail laid along the movement direction, a movable section mounted to the rail in a reciprocating manner along the movement direction relative to the rail, and a magnetic body mounted to the movable section.
A linear scale has a scale element provided on the movable section of the linear motor, and a sensor provided on the base frame. The linear scale is designed to obtain a distance between the scale element and a reference position based on a detection value of the sensor to detect a position of the movable section. Recently, a system has been developed in which a plurality of sensors are arranged over a stroke range greater than a length of a scale element.
For example, a system disclosed in JP 2005-143171A has a plurality of detection elements on a base frame. In order to determine as to which of the detection elements reads the scale element, each of the detection elements is equipped with a flip-flop circuit. A mechanism inputs a trigger signal into the flip-flop circuit corresponding to the detection element which is detecting the scale element, to latch the flip-flop circuit in an enable state. The mechanism then releases latch based on an output of a zero-point signal, thereby counting the read signal while selecting one of the detection elements which is currently reading, and a plurality of position signals output from the detection elements are synthesized together.
The flip-flop circuit in each of the detection elements, and the mechanism (AND gate, differentiation circuit, etc.) for latching the flip-flop circuit, drive up a cost. Meanwhile, it is not easy to accurately synthesize signals detected from whole range of an elongate linear motor to obtain a distance from a reference point of the linear motor, with a low-cost mechanism.
Accordingly, it is an object of the present invention to obtain an accurate distance from a reference point at a low cost.